It sounds like the victim you mention, Darla Scott, was exposed to the elements from the end of August to the beginning of November, so slightly longer than 2 months. It also sounds like she was not skeletonized. The same situation happened with Morgan Harrington, where she was not completely skeletonized and it was still possible to collect some DNA.
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The formula for determining the time for complete skeletonization is as follows:
y = 1285/x, where
y = number of days until skeletonization
x = average temp in Centigrade
(number of days until skeletonization) = 1285/(average temp in centigrade)
formula reference:
http://www.sgm.ac.uk/pubs/micro_today/pdf/110108.pdf
average temp graph:
http://www.weather.com/weather/wxclimatology/monthly/graph/50707
Using info from the linked average temp graph, I arrive at the average temperature of 52.6 F, or 11.4 C
y = 1285/11.4
The number of days until skeletonization in Evansdale: 112 days
The number of days that the children were missing (July 13 - December 5): 145 days.
There is no possibility that the children were not skeletonized if they were killed shortly after they were abducted, they were in that location for 5 months and they were "bones" (per Elizabeth's mother).